Topologically Induced Heterogeneity in Gradient Copolymer Brush Particle Materials

A facile synthetic route toward silica brush particles with random and gradient copolymer grafts comprised of n-butyl acrylate (BA) and methyl methacrylate (MMA) using surface-initiated atom transfer radical polymerization (SI-ATRP) was developed. The effect of chain architecture on the structure and thermomechanical properties of copolymer-tethered brush particle films was investigated and compared to linear copolymer analogues. Random copolymer brush systems displayed a uniform and narrow glass transition, and the dependence of the glass-transition temperature followed a similar trend as for linear copolymers. In contrast, gradient brush materials featured a splitting of the glass-transition temperature with increasing MMA content along with a step-like increase of Young’s modulus and a decrease in ductility. The results were interpreted in terms of the effect of brush architecture on the microstructure of films. The results suggested a “molecularly uniform” microstructure in films of the random brush as well as linear random and linear gradient copolymer systems. In contrast, gradient brush systems featured a heterogeneous microstructure in which MMA-rich regions acted as “traps” that reduced chain mobility. The formation of compositional heterogeneity was rationalized by the “prescribed orientation” of gradient copolymer chains near the particle interface that amplified composition fluctuations and hence local segregation of repeat units. The results highlight the need for a better understanding of the role of “geometric constraints” on the physical properties of copolymer brush particle-based hybrid materials and the opportunity for multiple property enhancement by control of segment distribution in addition to the overall composition.